This study aimed to identify any drug-related problems (DRPs) in diabetic patients during Ramadan fasting in Saudi Arabia. The study used a mixed-methods approach consisting of two phases and was conducted in Makkah, Saudi Arabia from December 2017 to March 2018. The first phase of the study involved qualitative semi-structured individual interviews with diabetic patients. A 13-item questionnaire was used in the second phase to further identify DRPs in the wider population. The data was mainly presented as frequencies and percentages. Inferential statistics was performed using Statistical Package for Social Sciences (SPSS) version 21 to compare relevant variables/questions using the chi-square test. Twenty patients (10 male, 10 female) attended face-to-face interviews during the first phase of the study while 95 (40 male, 55 female) completed the questionnaire in the second phase of the study. Two possible risk factors for DRPs were identified from the qualitative data: patient-related factors, including changes in their medicine intake during fasting, and healthcare professionals-related factors, including lack of advice from healthcare professionals regarding fasting. The quantitative results indicated that 52 (54%) of the 95 participants who observed fasting reported to have changed the way they were taking their medicines. Furthermore, 41% of the participants experienced general healthcare problems such as hypoglycemia, hyperglycemia, fatigue, excessive sweating, and gastrointestinal disturbances. Healthcare professionals need to educate patients who are at risk of DRPs by providing structured education and counseling.